The knee joint basically consists of the bone interface of the distal end of the femur and the proximal end of the tibia. Appearing to cover or at least partially protect this interface is the patella, which is a sesamoid bone within the tendon of the long muscle (quadriceps) on the front of the thigh. This tendon inserts into the tibial tuberosity and the posterior surface of the patella is smooth and glides over the femur.
The femur is configured with two knob like processes (the medial condyle and the lateral condyle) which are substantially smooth and which articulate with the medial plateau and the lateral plateau of the tibia, respectively. The plateaus of the tibia are substantially smooth and slightly cupped thereby providing a slight receptacle for receipt of the femoral condyles.
When the knee joint is damaged whether as a result of an accident or illness, a prosthetic replacement of the damaged joint may be necessary to relieve pain and to restore normal use to the joint. Typically the entire knee joint is replaced by means of a surgical procedure that involves removal of the surfaces of the corresponding damaged bones and replacement of these surfaces with prosthetic implants. This replacement of a native joint with a prosthetic joint is referred to as a primary total-knee arthroplasty.
On occasion, the primary knee prostheses fails. Failure can result from many causes, including wear, aseptic loosening, osteolysis, ligamentous instability, arthrofibrosis and patellofemoral complications. When the failure is debilitating, revision knee surgery may be necessary. In a revision, the primary knee prosthesis is removed and replaced with components of a revision prosthetic knee system.
Knee implant systems for both primary and revision applications are available from a variety of manufacturers, including DePuy Synthes Products, LLC of Warsaw, Ind. DePuy and others offer several different systems for both primary and revision applications. For example, DePuy Synthes Sales, Inc. offers the P.F.C. SIGMA® Knee System, the LCS® Total Knee System, and the S-ROM Modular Total Knee System. These orthopaedic knee systems includes several components, some appropriate for use in primary knee arthroplasty and some appropriate for use in revision surgery.
DePuy Synthes Sales, Inc. also offers other orthopaedic implant systems for other applications. One such system is the LPS System. The LPS System is provided for use in cases of severe trauma and disease. In such cases, the trauma or disease can lead to significant amounts of bone loss.
These systems include modular components, allowing the surgeon to construct an implant that meets the needs of the individual patient. Some combinations include modular stem extensions for use in conjunction with a distal femoral implant component, a proximal tibial implant component, or metaphyseal sleeves that may be used with such femoral and tibial implant components.
Although these prior knee implant systems have provided surgeons with great flexibility in meeting patient needs, and the modularity of these systems provides the opportunity to reduce the number of components needed in a surgical kit, these systems still require a substantial number of components.
In these assemblies of modular components, the fatigue strength of the components at their connections is important in ensuring the longevity of the assembly in use, and the characteristics of the connection components, and in particular, the dimensions of the connection components, are significant in ensuring that the assembly has an appropriate fatigue strength. However, the loading requirements for different assemblies may vary; for example, the loading requirements for the connection between a femoral stem connection may be greater than the loading requirements for a tibial stem connection. If the geometry of the connection of the tibial components satisfies the fatigue strength loading requirements for tibial loading, this same geometry may not satisfy the fatigue strength loading requirements for femoral loading. Moreover, if the connection includes a metaphyseal sleeve, the loading requirements may further differ. If the connections are optimized for the maximum anticipated loads, then the design may not meet other constraints; for example, design constraints for the tibial components may not allow for use of a connection that is optimum for the femoral components. Therefore, different components may be required for use on the femoral and tibial sides; for example, different stem configurations may need to be provided for use with the femoral and tibial components. Since multiple sizes of components are also required, duplicates of each size of stem may be required, substantially increasing the number of components in each set of implants provided to the surgeon.
Accordingly, a need exists for a modular prosthesis system that optimizes the fatigue strength of each modular connection while allowing for a more limited number of components required in each implant set provided to the surgeon.